The present invention refers to a system and process for rare earth extraction from monazite ore, bastnasite ore, ore beneficiation or industrial waste containing rare earth elements as oxide, phosphate, sulfate or carbonates.
Conventional leaching process for rare earth elements requires a large amount of acid, since some rare earth elements may be associated with iron and aluminum containing materials. Monazite and bastnasite ores and iron scrap containing rare earth elements are example of those types of materials, and its composition leads to high acid consumption, since the iron and aluminum are also responsible for acid consumption. These processes are often unfeasible.
Gupta, C. K. and Krishnamurthy, Extractive Metallurgy of Rare Earths, CRC press, disclose conventional rare earth processing options. The treatment with sulfuric acid is usually carried out in two stages, using 93% acid, temperatures between 210 and 230° C. and dissolving the rare earth sulfate by addition of water. With this method, depending on the acid/ore ratio, temperature and concentration, either thorium or rare earths may be selectively solubilized or both thorium and rare earths totally solubilized. Since the reaction is strongly exothermic, there is a limit to the addition of acid, but below 200° C. the reaction proceeds very slow. In the caustic process, after the reaction is processed, the mixture is cooled to 70° C. and diluted about ten times in cold water. Rare earths and thorium salts stay in solution while the insoluble portion quickly precipitates due to the high density. The phosphate content of the ore is recovered as a marketable by-product, trisodium phosphate, at the beginning of the flowchart, and this has been a major attraction for the commercial use of this process. In the usual industrial practice, fine ground monazite is attacked with 60-70% sodium hydroxide solution at 140-150° C. In the Soviet practice, the grinding and caustic digestion are performed in one step and this enables about 50% savings in caustic soda consumption. Krumholz (1957) achieved a clean separation by digesting the ore with caustic soda at 170° C. under a pressure of several atmospheres. The mixed rare earth thorium hydroxide cake is processed for rare earths and thorium recovery by a variety of methods. Document U.S. Ser. No. 13/887,027, in the name of Vale S. A, discloses a system and method for rare earth extraction wherein rare earth from iron containing minerals can be selectively extracted. However, this process requires a long residence time in the furnace, and usually does not provide satisfactory results for high silica containing ore.
Christoph Pawlik, Recovery of Rare Earth Elements from Complex and Low Grade Deposits, Uranium-REE conference, May 2013, ALTA 2013, Perth, Australia, (Uranium-REE Conference in Perth, Australia) discloses a review on the rare earth process, especially for low grade rare earth ore. Pawlik mentions the mineralogy dependence and several options for processing routes, proposing the conversion of rare earth in sulfates wherein the ore is mixed with concentrate sulfuric acid at temperatures between 150 and 250° C. However, this process requires a large amount of acid, due to the most of iron and aluminum are reacted into sulfates.
Patent FR2826667, cited as prior to U.S. patent Ser. No. 13/887,027 discloses a first stage leaching with sulfuric acid is added to the ore, followed by calcination steps/burning (roasting) conducted at elevated temperature. Precipitation steps/extraction/isolation were also described in this document. This invention has several drawbacks, as a high acid consumption and the need of very high temperatures. The mechanism involved comprises the conversion all species in the ore into sulfates (including the impurities) and decomposing them at high temperatures into insoluble oxides, releasing SOx. This invention takes care of reducing the amount of impurities added but still needs a high amount of sulfuric acid to be added and high temperatures to decompose some of the formed sulfates. Unlike the present invention which has a lower acid consumption, lower temperature and furnace time.
Further the above-mentioned documents fail on presenting the process condition, selective pyrolysis and reactions mechanism for better result control.
The present invention provides a system and process aiming to reduce the furnace residence time, increase the performance on high silica containing ore, allowing the recovery of sulfur during pyrolysis stage and decreasing the sulfuric acid consumption. The controlled mixture stage allows rare earth conversion at low temperature, which is relevant for high silica containing ore, therefore increasing rare earth extraction and decreasing furnace time. SO3 is released during selective pyrolysis stage, increasing the conversion of iron and aluminum oxide in sulfates, enhancing the encounter probability between ion/aluminum sulfates and rare earth compound, increasing rare earth sulfate conversion and extraction. The pyrolysis products are easily leached, since most of them are oxides.